Nickel-chromium-cobalt alloys

ABSTRACT

NICKEL-CHROMIUM-COBALT-BASE ALLOYS CONTAINING CORRELATED AMOUNTS OF TITANIUM, ALUMINUM, COLUMBIUM AND MOLYBEDENUM AS WELL AS CARBON AND OTHER CONSTITUENTS OFFER A COMBINATION OF HIGH TEMPERATURE STRESS-RUPTURE STRENGTH, DUCTILITY AND IMPACT RESISTANCE TOGETHER WITH GOOD CORROSION RESISTANCE OF SUCH MAGNITUDE AS TO RENDER THE MATERIALS SUITABLE FOR VARIOUS GAS TURBINE ENGINE COMPONENTS.

nited States 3,723,108 NICKEL-CHROMIUM-COBALT ALLGYS Peter Lindsay Twigg and Philip James Terry, Halesowen,

England, assignors to The International Nickel Company, Inc., New York, N.Y.

Filed Mar. 4, 1970, Ser. No. 16,367 Claims priority, application Great Britain, Mar. 7,, 1969, 12,260/69 Int. Cl. CZZc 19/00 U.S. Cl. 75-171 Claims ABSTRACT OF THE DISCLOSURE As those skilled in the art are aware, research efforts have been endless in the quest of alloys capable of delivering in use certain metallurgical properties. Usually such alloys have been of the nickel-chromium or cobaltchromium-base variety (rather exotically termed super alloys) and have found extensive utility in a number of high temperature applications. Commonly these alloys contain one or more such elements as titanium, aluminum, columbium (niobium), molybdenum, etc, primarily for the purpose of imparting strength and hardness properties.

However, such characteristics as strength and hardness, including elevated temperature stress-rupture strength, are by no means all encompassing in respect of the properties such alloys might be called upon to exhibit. Resistance to high temperature corrosion, for example, is often an indispensible necessity, depending, of course, upon the intended application. And this aspect plus other considerations, e.g., tensile ductility, the ability to absorb impact energy upon prolonged exposure, etc., focus attention on the objectives of the subject invention since such properties usually do not go hand in hand.

It has been demonstrated, for example, that with increas ing chromium content the ability of various nickel-chromium-cobalt-base alloys to resist corrosion at high temperatures, in, say, sulfur-containing environments is significantly improved. But it has also been shown that increasing the chromium level is not Without its drawbacks. For example, in such alloys high percentages of chromium have been known to induce loss in stress-rupture life. Another adverse effect can be a decrease in the amount of the hardening and strengthening elements titanium, aluminum, columbium and molybdenum that may be present without causing alloy embrittlement upon prolonged exposure to high temperatures. Accordingly, the present invention is directed in the main to minimize these conflicting effects.

In any case, it has now been discovered that an excellent combination of high temperature stress-rupture strength, ductility and impact strength can be attained together with good corrosion resistance, illustratively in respect of hydrocarbon fuels containing sulfur or in marine environments in which chlorides might be injected into, say, gas turbine engines, with high chromium, nickel-base alloys containing special amounts of titanium, aluminum, columbium, molybdenum and other elements as described herein.

Generally speaking and in accordance with the present invention, alloys contemplated herein contain (in weight 3,723,108 Patented Mar. 27, 1973 percent) from 23.5% to 26% chromium, about 0.01% to 0.2% carbon, about 10% to 24% cobalt, from 4.25% to 5.6% in total of titanium plus aluminum, the ratio of titanium to aluminum being from about 1:1 to 4:1, from about 0.25% to 2% columbium, with the proviso that the total percentage of titanium plus aluminum is so related to the percentage of columbium that it advantageously corresponds to a point in the area BCDEB in the accompanying drawing, from about 0.5% to 2% molybdenum, from 0.001% to 0.05% boron, e.g. 0.001% to 0.01% or 0.02%, up to 0.15% zirconium, with the further proviso that ten times the percentage of boron plus the percentage of Zirconium is at least 0.02%, up to about 0.1% hafnium, up to about 0.04% magnesium, up to about 0.3% rare earth metal, up to about 2% yttrium, and the balance, apart from impurities, being essentially nickel.

In carrying the invention into practice, the minimum chromium content of 23.5% is dictated by the need for the greatest corrosion-resistance, but more than about 26% leads to embrittlement or loss of stress-rupture strength or both. In striving for the best combination of results the chromium should be from 24% to 25%.

If the percentage of carbon is below 0.01%, stressrupture strength is reduced. On the other hand, too much carbon renders the alloys susceptible to embrittlement, and it should therefore not exceed 0.2%. A range of 0.015% to 0.08% is most beneficial.

Cobalt strengthens the alloys and at least 10%, preferably at least 12%, and most advantageously at least 14% is required for this purpose. Should the cobalt much exceed 24% the alloys tend to undesirably embrittle on prolonged heating and this tendency can be lessened with cobalt levels that do not extend beyond 22%.

The alloys are further and principally strengthened by titanium, aluminum, columbium and molybdenum. If the combined titanium and aluminum content is less than 4.25%, stress-rupture life is inadequate. Moreover, having regard to the columbium content the impact strength after prolonged heating at 850 C. is also inadequate if the combined titanium plus aluminum is too great. This is illustrated by the point lying above and to the right of the line AC in the accompanying drawing, in which the sum of the titanium plus aluminum is correlated with (and plotted against) columbium. Stressrupture life is generally improved by the presence of columbium, and advantageously the alloys contain at least 0.25% and most beneficially at least 0.5%. However, it the percentage of columbium exceeds about 2% the alloys have unnecessarily low stress-rupture lives and low room temperature impact resistance. The amount of columbium can be reduced below 0.25 and down to the zero level provided that it is correlated with the titanium plus aluminum as to represent a point within the area ABEFA of the drawing.

The above described titanium, aluminum and columbium effects are illustrated by the results of tests on a series of alloys having the compositions set forth in Table I, of which only Alloys Nos. 1 to 4 are in accordance with the invention. The TizAl ratio in each alloy was 2:1. In addition to nickel and the constituents given, each alloy nominally contained 0.003% boron and 0.05% zirconium. The alloys were vacuum-melted, an addition of 0.03% magnesium being made as a Ni-15% magnesium at 1150 C., air cooled, and then exposed for a period of 1000 hours at 850 C.

In the last three columns of Table I are set forth the stress-rupture life in hours at 17 ton-f./in. at 815 C., the percentage elongation at rupture, and the Charpy V- Boron and to a lesser extent zirconium both improve the stress-rupture strength, and the alloys must contain at least about 0.001% and preferably at leastabout 0.003% boron but not more than about 0.05% boron. Above about 0.05% boron the forgeability of the alloys notch impact strength in foot-pounds determined at room 1s detrimentally affected. ZlI'COIllUlTl can be present 1n temperature. The four alloys tested whose compositions amounts up to 0.15%, and the combined content of boron were within the area ACDFA (Alloys l-4) all had stressand zirconium, as expressed by 10 B+percent Zr should rupture lives in excess of 280 hours and impact values be at least about 0.02%. about 10 ft.-lbs., while each of the other alloys was in- 10 The advantage of a boron content of at least about ferior in one or the other of these properties. 0.003% is illustrated by the results in Table III which TABLE I Composition (percent by weight) Stress-rupture Impact Lite Elong. strength Alloy No. C Al Ti Ti+Al C1 C Cb M0 (hr.) (percent) (it.-lbs.)

A 0. s 1. 3 2. 7 4. 0 25.1 20 2. 05 143 9. 2 33. 3 1 0. 047 1. 45 3.2 4. 65 25.2 21. 2 2. 0 337 0. 3 16.8 z 0. 047 1. s 3.5 5. 3 24.8 21.1 2. 0 321 5. 5 13.7 3. 0. 046 1. 05 3. 2 4. 85 24.4 19.8 0. 0 2.1 283 14. 5 13 0. 047 1. s 3. 55 5. 35 24.5 19.8 0. 0 2. 05 313 9. s s. 0 c 0. 040 1.3 2.3 3.5 24.4 20.1 1.1 2.1 170 0.4 28.9 D 0. 050 1. 35 2. 7 4. 05 24. 8 0.98 2. 05 157 14. 9 23. e 4. 0. 043 1. 5 3. 05 4.55 25.1 19. 7 1. 05 2.1 354 0. s 15. 0 E 0. 044 1. 2. 3. 5 24.1 20.1 2. 1 2.15 197 6. 5 17. 4 F 0. 05s 1. 35 2. 7 4. 05 24.2 19. 7 1. 9 2. 05 214 0. 4 13. 7 (1 o. 045 1. 55 3. 0 4. 55 23. 5 10. s 2. 05 2.05 215 5. 0

The alloys in Table I are represented by the points plotted on the accompanying drawing. In each case the figures in parentheses are the stress-rupture life, the elongation, and the impact strength.

Apart from the foregoing data, it should also be mentioned that ratios of titanium to aluminum less than 1:1 lead to loss of stress-rupture ductiliy and a lowering of impact resistance while if the ratio exceed 4:1 the stressrupture strength is inadequate. Preferably the ratio is from 1:1 to 2.5:1.

were obtained for alloys nominally containing, in addition to chromium, molybdenum and boron in the amounts shown, 0.04% carbon, 20% cobalt, 3% titanium, 1.5% aluminium, 1% niobium, 0.04% zirconium, balance, apart from impurities, nickel.

The alloys were prepared, heat-treated and tested as described with reference to Table I, but with additional stress-rupture property determinations being carried out at 14 ton-f./in. and 815 C.

TABLE III Stress-rupture properties 17 ton-L/in. 14 ton-L/lnJ Cr Mo )3 Life Elong. Sigma Lite Elong. Sigma Strength Alloy N (percent) (percent) (percent) (hrs (percent) present (hrs.) (percent) present (IL-lb.)

25 2 0. 003 268 B No 707 6 Yes 16. 9 25 1. 5 0.003 624 8 N o 1, 107 11 No 13. 7 24 1. 5 0.003 333 5 N0 944 0 No 24. 5 1. 6 0. 012 448 6 N0 1, 310 5 N0 10. 6 26 2 0. 003 347 8 No 967 11 Yes 4. 3

In the absence of molybdenum, the alloys lack stressrupture strength, and at least 0.5% molybdenum must be present. As the molybdenum content increases, the stressrupture life increases up to about 2% molybdenum and then decreases slightly, but the impact strength after prolonged heating at 850 C. decreases progressively with increasing molybdenum contents. Though the molybdenum might be as high as 2.2% contents above about 2% tend to give rise to the risk of sigma-phase formation. Most beneficially, the molybdenum content is from 0.5 or 1% to 2%. The molybdenum effect is shown in Table H concerning alloys nominally containing, in addition to molybdenum, titanium and aluminium in the amounts shown, at a TizAl ratio of 2:1, 0.04% carbon, 25% chromium, 20% cobalt, 0.003% boron, 0.05% zirconium, 0.02% magnesium, the balance being nickel and impurities. The alloys were prepared, heat-treated and tested as described in connection with Table I. Alloy No. 2 is in accordance with the invention, but Alloys H and K are not.

It can be seen that with a boron content as low as 0.003% and a molybdenum content as high as 2% and a chromium content as high as 26%, there may be a tendency for embrittlement to occur on prolonged exposure at 815 C. and 14 ton-f./in. Accordingly, for maximum freedom from embrittlement on prolonged exposure at high temperature and stress, the boron content should be at least 0.003%, the molybdenum content less than 2% and the chromium content less than 26%.

The resistance of alloys according to the invention to corrosion by the combustion products of impure hydrocarbon fuels and by marine salts has been determined by tests in which specimens were exposed to a molten mixture of 25 by Weight of sodium chloride and 75% sodium sulfate at 900 C. The corrosion damage was assessed by comparing the weight of each specimen, after removing the corrosion products by cathodic descaling in molten sodium hydroxide, with the initial weight before exposure.

The tests were performed in two ways:

Test A: Samples of each alloy were half-immersed in the salt mixture while heated in air.

Test B: Samples of each alloy were heated in a vertical open-top furnace into which the salt mixture was continuously fed as a fine dispersion at a rate of 5 g./hour.

The results obtained for Alloy 4 are set in Table IV, the results being generally representative of alloys in accordance herewith.

TABLE 'IV Weight loss (mg/cm):

Alloy No. 4 Composition (wt. percent) C 0.043 Cr 25.1 C0 19.7 Mo 2.15 Ti 3.05 Al 1.50 Cb 1.05 Test A:

300 hr Test B:

3 hr. hr. 27

Balance nickel, impurities and nominally 0.05% Zr and 0.003% B.

As to the presence of other elements, hafnium can be present in amounts up to 0.1%, for example, from 0.02% to 0.07%, to improve the weldability of the alloys, especially those containing both boron and zirconium. Magnesium is advantageously added in amounts up to 0.04% to improve workability, but larger amounts have the opposite effect rendering working more diflicult. Most suitably the magnesium content is from 0.01% to 0.03%.

The resistance of the alloys to oxidation and scaling is improved by the presence of rare earth metals, and one or more of these metals can be added, for example in the form of Mischmetall. From 0.01% to 0.3% of rare earth metal, e.g. from 0.03% to 0.08%, has distinct benefit. We find that yttrium additions also improve the oxidation and scaling resistance of the alloys and also their resistance to sulfidation. Thus, yttrium can advantageously be added in amounts from 0.2% to 2%, for example, from 0.5% to 1%.

Of the elements that may be present as impurities, silicon has a deleterious effect on corrosion-resistance and should therefore be kept below 1% and preferably below 0.5%. Other impurities may include manganese in amounts up to 1% and iron in amounts up to 2%. (Tantalum may be introduced incidentally with columbium in an amount up to about one-tenth thereof. For purposes herein, such amounts of tantalum are to be regarded as part of the columbium content.)

A particularly advantageous combination of properties is exhibited by alloys containing from 24% to chromium, 19% to 22% cobalt, 0.03% to 0.06% carbon, 2.8% to 3.2% titanium, 1.4% to 1.6% aluminum, 0.5% to 1.0% columbium, 1.8% to 2.0% molybdenum, 0.001% to 0.006% boron, 0.03% to 0.06% zirconium, up to 0.03% magnesium, up to 0.07% hafnium, up to 0.3% rare earth metal and up to 1% yttrium, the balance, apart from impurities, being nickel. Other advantageous alloys contain from 14% to 17% cobalt, the balance of their composition being as set forth above.

Further advantageous alloys contain from 19% to 22% cobalt, 1.4% to 1.6% molybdenum and 0.010% to 0.015% boron, the balance of their composition being as set forth above. A particularly preferred alloy has the nominal composition 24.5% chromium, 20% cobalt, 1.5% molybdenum, 3% titanium, 1.5% aluminum, 1% niobium, 0.04% zirconium, 0.012% boron, 0.04% carbon, balance nickel apart from impurities.

To develop the full stress-rupture properties of the alloys in wrought form they must be subjected to a heat treatment comprising solution heating and subsequent aging. The solution treatment may consist of heating from 1 to 8 hours in the temperature range of 1050 C. to 1250" C., and the alloys may then be aged by heating for 1 to 24 hours in the temperature range of 600 C. to 950 C. An intermediate aging treatment consisting of heating for 1 to 16 hours at 800 C. to 1050 C. may

6 be interposed between the solution treatment and the final aging stages. The alloys may be cooled at any convenient rate after each heat treatment stage, e.g., by air cooling (generally to room temperature) or by direct transfer from a furnace at one temperature to one at a lower temperature.

The alloys can be air melted, but to ensure the best creep properties they are preferably melted and cast under vacuum. They can be readily processed by conventional means such as extrusion, forging or rolling. Although they are primarily intended for use in the wrought form as gas turbine blades, they are suitable for use in other applications where a combination of good-stressrupture strength and resistance to corrosion is required, particularly for articles and parts that are subject in use to stress at high temperatures while exposed to the combustion products of impure hydrocarbon fuels or to salt or both. They may also be used to make cast articles and parts, which may be used with or without heat treatment.

The alloys of the invention are also useful as matrix materials for alloys dispersion-hardened by the presence of finely divided refractory particles such as thoria, yttria, lanthana, ceria, or rare earth oxide mixtures, such as didimia. The refractory compound may suitably be present in an amount of at least 0.2%, preferably 0.5 to 5%, by volume and the particles should preferably be maintained as fine as possible, for example below 0.5 micron, most suitably from 10 angstroms to 1000 angstroms (0.001 to 0.1 micron). The present invention includes the use of the alloys as matrix materials in dispersionhardened alloys.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. A nickel-chromium alloy characterized by a combination of good stress-rupture strength, tensile ductility and the ability to absorb impact energy together with resistance to various corrosive media, said alloy consisting of from 23.5% to 26% chromium, about 0.01% to about 0.2% carbon, from 10% to 24% cobalt, from 4.25% to 5.6% in total of titanium plus aluminum, the ratio of titanium to aluminum being from 1:1 to 4:1, from 0.25% to 2% columbium with the proviso that the total percentage of titanium plus aluminum is correlated with the columbium so as to represent a point within the area BCDEB of the accompanying drawing, from 0.5% to 2.2% molybdenum, from 0.001% to 0.05% boron, up to 0.15% zirconium, the sum of 10 percent B+ percent Zr being at least 0.02%, up to 2% iron, up to 1% silicon, up to 1% manganese, up to about 0.1% hafnium, up to 0.04% magnesium, up to 0.3% rare earth metal, up to 2% yttrium, and the balance essentially nickel.

2. An alloy in accordance with claim 1 containing 24% to 25% chromium, at least 14% cobalt, magnesium present in an amount up to 0.03%, from 0.001% to 0.003% boron, up to 0.5% silicon, the alloy being characterized by a stress rupture strength on the order of 280 hours or higher at a temperature of about 815 C. under a stress of 17 ton-f./in. while concomitantly exhibiting an impact strength at room temperature of at least 10 ft.-lbs.

3. An alloy in accordance with claim 1 in which the ratio of titanium to aluminum is from 1:1 to 2.5 :1.

4. An alloy in accordance with claim 2 containin 0.015% to 0.08% carbon, 14% to 22% cobalt, and 0.5% to 2% columbium, the alloy being characterized by a stress rupture strength on the order of 280 hours or higher at a temperature of about 815 C. under a stress of 17 ton-f./in. while concomitantly exhibiting an impact strength at room temperature of at least 10 ft.-lbs.

5. A nickel-chromium alloy characterized by a stress rupture strength on the order of 280 hours or higher at a temperature of about 815 C. under a stress of 17 ton-f./in. while concomitantly exhibiting an impact strength at room temperature of at least 10 ft.-lbs. together with good resistance to various corrosive media, said alloy consisting of from 23.5% to 26% chromium, about 0.01% to about 0.2% carbon, from 10% to 24% cobalt, from 4.25% to 5.6% in total of titanium plus aluminum, the ratio of titanium to aluminum being from 10 1:1 to 4:1, up to 2% columbium with the proviso that the total percentage of titanium plus aluminum is correlated with the columbium so as to represent a point within the area ACDFA of the accompanying drawing,

5 essentially nickel.

References Cited UNITED STATES PATENTS 2,570,193 10/1951 Bieber et al. 75-17l 3,479,157 11/1969 Richards et al. 75-171 3,516,826 6/1970 Ward et al. 7517l RICHARD O. DEAN, Primary Examiner 

